The present invention relates to an arrangement for damping oscillations or pulsations in general, and more particularly to an arrangement of this type which is especially suited for use in an apparatus for pumping liquid fuel from a storage tank to an internal combustion engine of a motor vehicle.
There is already known a cushion-like damping element, in which the body of oscillation-damping gas is contained in and surrounded by a single bag-shaped diaphragm member. This damping element is capable of damping oscillations in the range of .+-.0.4 bar to satisfaction when used in displacement pumps. Such oscillations or pressure pulsations repeat themselves in a roller cell pump utilized as a fuel pump as often as there are mutually separated chamber volumes. Thus, in a five-cell roller cell pump, such pressure pulsations occur five times during each revolution. Each of these pumping operations is accompanied by a short-lived reduction of the negative suction pressure and a temporary increase of the pumping or output pressure. The so obtained pressure oscillations of the medium being pumped, which come into existence at the suction side as well at the pressure side of the pump and the frequency of which is determined by the rotational speed of the pump, propagate in the form of sound waves (primarily as solid-borne vibrations) and, under certain circumstances, result in a relatively high noise level of the pump.
When such noise-generating pump is a fuel pump employed in a motor vehicle or another similar mobile unit, then the main source of noise can be found in the fact that the suction and pressure side pressure variations are transmitted to and carried as solid-borne vibrations by, on the one hand, the chassis or body (on the pressure side of the pump) and, on the other hand, the fuel tank (at the suction side of the pump). These parts of the motor vehicle or other mobile unit form resonance bodies, by means of which the noise generation is considerably enhanced. From this, there results the demand for damping the oscillations as close to their source as possible or feasible. However, the aforementioned conventional damping element has the disadvantage that it can be damaged or destroyed during the fluid-tightness testing of the pumping unit when it is accommodated in the interior of such a pumping unit during the testing operation. Such testing is necessary especially when the pumping unit is not to be mounted in the interior of the storage tank, but when it is to be mounted externally thereof. The testing pressure employed in testing the pumping unit for fluid-tightness, which is usually in excess of approximately 6 bar, is well above the pressure at which the conventionally constructed damping element as discussed above retains its stability or, in other words, the strength of the diaphragm constituting the same. Hence, the conventional damping element is usually destroyed during the testing operation and hence cannot perform its damping function during the operation of the pump.